The oxidation state of Ti in hornblende and biotite determined by electron energy-loss spectroscopy, with inferences regarding the Ti substitution

Max T. Otten, P R Buseck

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Abstract

Structural formulae of many Ti-rich hornblendes and biotites exhibit cation deficiencies that increase with Ti content. These deficiencies may be caused by the presence of trivalent instead of tetravalent Ti, of oxygen replacing hydroxyl, or of vacancies on octahedral cation sites. In order to determine the oxidation state of their Ti, electron energy-loss spectra of natural, Ti-rich hornblende and biotites are compared with spectra of natural and synthetic Ti-bearing oxides and silicates. Spectra of Ti2O3 and TiO2 demonstrate that the Ti L2,3 edge for Ti3+ is shifted by ca. 2 eV to lower energy relative to the edge for Ti4+. Oxidation states of Ti determined by energy-loss spectroscopy for several minerals agree with data from other techniques: tetravalent for Ti-omphacite, perovskite, ilmenite and titanite; trivalent for NaTi3+Si2O6 pyroxene and fassaite from the Allende meteorite. The energy-loss spectra of the hornblende and biotite show that their Ti is tetravalent and cannot be the cause of the cation deficiencies. The relations between Ti contents and the number of deficiencies differ for hornblende and biotite. Hornblende shows a 2:1 relation between Ti content and deficiencies, suggesting Ti is coupled to an oxygen that is replacing hydroxyl (Ti-oxyhornblende substitution). Biotite shows a 1:1 relation, consistent with coupling of Ti to a vacant octahedral cation site (Ti-vacancy substitution) or to a variable combination of such vacancies and two oxygens replacing hydroxyls (Ti-oxybiotite substitution).

Original languageEnglish (US)
Pages (from-to)45-51
Number of pages7
JournalPhysics and Chemistry of Minerals
Volume14
Issue number1
DOIs
StatePublished - Jan 1 1987

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ASJC Scopus subject areas

  • Materials Science(all)
  • Geochemistry and Petrology

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